Increasing attention is being paid to the chirality of nanomaterials, and focus is moving toward the application of nanostructural analyses, such as conformational studies of biomolecules upon interacting with nanoparticles, information about nanocarriers, analysis of DNA biofunction on nanoparticles, and nanomaterial assemblies. Among these areas, nanomaterial assemblies endowed with fascinating optical properties may be regarded as one of the most active fields of research. In optical spectroscopy, circular dichroism (CD) can be produced by molecular-recognition-driven assembly of plasmonic nanoparticles into chiral structures. Alivisatos and co-workers proposed the concept of chiral plasmonic nanostructures of Au NPs with tetrahedral symmetry, and more importantly, chiral enantiomers were successfully built by using four different sized Au NPs at the tips of a discrete pyramid. Tang's group has demonstrated that remarkable plasmonic CD signals could be produced in the visible light region based on the assembly of gold nanorods (Au NRs) and DNA hybrids. Govorov et al. proposed a theory to account for a plasmonic mechanism of optical activity in chiral complex assemblies composed of plasmonic nanoparticles. Their research demonstrated that the CD signal was very sensitive to the geometry. Our group has constructed multimeric assemblies of various geometries (ranging from dimers, trimers, and tetramers to very complex agglomerates) to achieve chiral assemblies with strong chiral optical activity by performing polymerase chain reaction (PCR) on the surfaces of Au NPs functionalized with primers. This unique optical property holds great promise for the fabrication of negative refractive index materials. Furthermore, the strong CD signal might also be exploited in the construction of smart sensors. Recently, research has been directed towards the detection of targets by utilizing the CD signal. For example, pairs of “right-handed” and “left-handed” molecules have been discriminated based on their interactions with chiral nanoparticles. Chiral Ag-1-cysteine complex nanoparticles have been prepared as a probe for Hg2+; the displacement of Ag+ from the chiral nanoparticles by Hg2+ induced a conformational change of the ligands on the surface of the NPs. In order to achieve specific detection using this unique property, a universal model based on specific molecular recognition is needed. Seeking a suitable model of molecular recognition is of tremendous importance for devising a chirality detection platform.
Immuno-recognition has long been among the most popular affinity-based recognition targets. A wide variety of immunosensors based on antibodies has been reported, such as those for small organic molecules, proteins, viruses, bacteria, and metal ions. The main advantage of the use of antibodies as recognition targets is their sensitivity and selectivity. Moreover, many antibodies have become commercially available.